Avoid changing overlay committed layer. (#5839)

* Avoid changing overlay committed layer.

* basic test

* Add some tx in the test.

* only update from backend value on missing entry in both layer.
deleted entry is replace by empty vec.

* test and review changes

* additional test and review change

* remove test on changing existing value, it does not always panic
depending on existing content

* Update primitives/state-machine/src/overlayed_changes.rs

* Update primitives/state-machine/src/overlayed_changes.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>